1. Field of the Invention
The present invention relates to a process for recovering unreacted materials in the effluent from a synthesis of urea from carbon dioxide and ammonia and, more particularly, it relates to an improved process for recovering unreacted materials from a urea synthesis effluent while reducing the amount of moisture entrained in the unreacted ammonia and carbon dioxide separated from the urea synthesis effluent.
2. Description of the Prior Art
In the production of urea, a total solution recycle process is well known which comprises reacting carbon dioxide with ammonia under high temperature and high pressure conditions, conventionally known and recognized by those skilled in the art as urea synthesis temperature and pressure conditions, subjecting the resulting urea synthesis effluent to a plurality of stripping or distillation stages under respective stepwise reduced presures to separate unreacted materials in the form of a gaseous mixture of ammonia, carbon dioxide and water vapor in each of the respective stages, absorbing the gaseous mixture of unreacted materials discharged from the low pressure distillation stage in an absorbent, increasing the pressure of the resulting absorbate stepwise for use as an absorbent for the mixed gas separated in a higher pressure decomposition stage, and recycling to the urea synthesis zone the absorbate discharged from the final highest pressure decomposition stage. The urea synthesis reaction, i.e., the reaction of ammonia with carbon dioxide to form urea and water, is a reversible reaction, so that the yield of urea decreases with an increase of water content in the urea synthesis reaction system. In order to improve the yield of urea, it is necessary to reduce to as small as possible the water content in the absorbate recycled to the urea synthesis zone. For this purpose, the absorption should be conducted under high pressure in the respective absorption stages and by use of a minimum amount of absorbent. Further, the amount of water which is evaporated and entrained in the gaseous mixture of unreacted materials separated in the respective separation stages should preferably be minimized for preventing the absorbate from being diluted. The gaseous mixtures separated in the separation zones under different pressure conditions have different water contents. Of these, the gaseous mixture which is separated in the low pressure separation zone operated under a gauge pressure of 0 - 5 kg/cm.sup.2 has the greatest content of water. Especially when the unreacted materials in the low pressure stage are stripped off with carbon dioxide fed into the bottom of the separation zone of the low pressure decomposition stage in order to completely separate the unreacted materials from the urea synthesis effluent, the water or moisture content in the separated gaseous mixture disadvantageously increases by an amount of water vaporized and entrained in the carbon dioxide. Accordingly, an additional means is required to suppress this increase in water content.
In order to overcome the above disadvantages, there has been proposed in U.S. Pat. No. 3,725,210 a process wherein the temperature at the top of the low pressure rectification zone having a gauge pressure of 0 - 5 kg/cm.sup.2 is maintained at 60.degree. - 120.degree. C. and the temperature at the bottom thereof is kept at 100.degree. - 140.degree. C. The present invention contemplates providing an improvement in the above process. In the known process, the temperature at the top of the rectification zone is maintained in the range of 60.degree. - 120.degree. C. so that it is possible to lower the vapor pressure and to reduce the water content in the gaseous mixture. In order to maintain the top of the rectification zone at such a low temperature, however, the gaseous mixture exhausted from the top of the rectification zone must be fed to a reflux condenser wherein the gaseous mixture is cooled to a temperature of 50.degree. - 100.degree. C. to condense the water vapor therein, the condensed water being recycled to the top of the rectification zone. Thus, the heat energy removed upon the reflux condensation results in a loss, increasing the quantity of heat required for heating the separation zone by an amount corresponding to said heat loss.